A damping force variable damper of the above type includes a cylinder having first and second fluid chambers, a piston separating the first fluid chamber and the second fluid chamber from each other, and a piezoelectric body operable to vary a damping force by controlling a fluid passage through which the first and second chambers communicate with each other, as is known in patent literature 1 below.
As for a damping force variable damper disclosed in patent literature 1, a piston defines a sealed chamber for increasing an amount of deformation of a piezoelectric body. The sealed chamber includes an enlarged-diameter passage in which a first piston member is slidably disposed and a reduced-diameter passage in which a second piston member is slidably disposed.
The piezoelectric body presses the first piston member to cause a viscous fluid (hereinafter referred to as “hydraulic oil”) in the sealed chamber to press the second piston. In so doing, an amount of displacement of the first piston member (i.e., the amount of deformation of the piezoelectric body) is increased in the sealed chamber for moving the second piston by the increased amount of displacement of the first piston member. When the second piston member moves, a valve member moves to control a fluid passage so as to vary a damping force.
The damping force variable damper of patent literature 1 includes an O-ring disposed on the first piston member for sealing the chamber. The O-ring disposed on the first piston member prevents the hydraulic oil in the sealed chamber from leaking toward the piezoelectric body. The damper further includes a return spring for returning the first piston (i.e., the piezoelectric body) to an original, non-displaced position when application of a voltage to the piezoelectric body is cut off.